Cephalostat radiographs of five untreated adult hydrocephalic patients were studied in order to explain events related to increased intracranial pressure and to differentiate them from those following shunt treatment. The hypophyseal fossa appeared to be well formed in every patient and no shallow fossa was found. The tip of the dorsum sella was rounded. The distances from the nasion to the sella region were increased, but those from the basion were decreased. The hypophyseal fossa was depressed inferiorly. The cranial base angle was more obtuse than in normal subjects or shunt-treated patients. The findings seem to indicate that a shallow. J-shaped sella results from the combined effect of the hydrocephalic condition and the shunt treatment, which cause a shortening of the anterior cranial base and move the fossa upwards and posteriorly away from the sphenoid bone.